Most people who have joined the unprecedented effort are already well known for their philanthropy, such as New York Mayor Michael R. Bloomberg; the eBay founder Pierre Omidyar and his wife, Pam; and CNN founder Ted Turner.

But there are few less-recognizable names from the giving world—for example, John Arnold, an energy investor in his mid-30s, and his wife Laura—as well as some prominent people who give but are better known for their business pursuits.

They include the media executive Barry Diller and his wife, the designer Diane von Furstenberg, and Ronald O. Perelman, the investor.

Surprising Progress

In a conference call last week discussing the pledge, Mr. Buffett said he and the Gateses have contacted between 70 and 80 people so far—and that their success rate, of roughly 50 percent, has been heartening.

“I’ve been surprised by how much progress we’ve actually made in the last six weeks,” said Mr. Buffett. “And I hope we can come back in six months and surprise you some more, and at least surprise ourselves.”

Joining Mr. Buffett on the call with reporters were Mr. Bloomberg and Thomas Steyer, an investment banker, and his wife, Kat Taylor.

Mr. Bloomberg, whose net worth Forbes pegs at $17.5-billion, said a person needs only so much and that it is important to not leave children “so much money that it ruins their lives.”

Mr. Bloomberg said he has gradually moved from giving anonymously—which he joked led him to get credit for gifts he didn’t even give—to donating publicly because he felt it was a more effective way to encourage generosity among others.

'We Don’t Give Up’

Mr. Steyer, who, with Ms. Taylor supports the causes of energy, good banking, and healthy food, said he thinks Mr. Buffett and the Gateses are “changing the face of American business.”

“Right now, when I look around, I think businesspeople and financial people are pretty widely mistrusted and seen as overwhelmingly self-interested,” he said.

Mr. Steyer said the Giving Pledge advances a different model of capitalism by which businesspeople “are not just laboring for themselves or for their families, but they have bigger responsibilities and belong to a wider community.”

The Gateses and Mr. Buffett have asked Giving Pledge members to write a letter describing their thinking on philanthropy, and many have. (The letters, and more information on the pledge, are online at http://givingpledge.org). The businessman George B. Kaiser says guilt first motivated him to give money away; the financier Kenneth G. Langone praises Mr. Buffett for “making this a national calling.” The letter from Larry Ellison, founder of Oracle, addressed “to whom it may concern,” explains his reasons for publicizing his philanthropy now after quietly giving hundreds of millions away already: “Warren Buffett personally asked me to write this letter because he said I would be 'setting an example’ and 'influencing others to give. I hope he’s right.”

Many nonprofit officials are less muted in their enthusiasm than Mr. Ellison. They hope the pledge will lead to billions of additional dollars being contributed to groups like theirs.

While it may prove impossible to assess the pledge’s impact, the net worth of donors who have so far committed to giving away a share of their wealth totals more than $200-billion, according to estimates of the donors’ assets published by Forbes magazine.

Some of the new philanthropy may come from outside the United States. Mr. Buffett said he and Mr. Gates will be meeting with some of China’s wealthiest people in late September and that in March, they will be visiting India.

Asked why some wealthy people have turned him down, Mr. Buffett cited a mix of reasons. Some prefer to keep their giving private—although Mr. Buffett says he has been successful in convincing a few people who showed that concern to overcome it and sign the pledge.

Some people spouted off about not liking the government, some gave him a lecture, some said they felt a responsibility to pass wealth along to their children, and others simply had a plane to catch, said Mr. Buffett.

He and fellow members of philanthropy’s most exclusive club still have plenty of fund raising to do, however, and they will try to gently nudge others to give during more phone calls and at dinners to be held later this year.

“We don’t give up on them,” he said. “Every saint has a past and every sinner has a future.”